The present invention relates to an apparatus and a method for controlling discharge capacity of a variable displacement compressor of an automotive air conditioner.
Generally, a refrigerant circuit of an automotive air conditioner includes a condenser, an expansion valve, an evaporator, and a compressor. The compressor draws and compresses refrigerant gas from the evaporator and discharges the refrigerant gas to the condenser. The evaporator transfers heat to refrigerant passing through the refrigerant circuit from air flowing inside a vehicle. Since the heat of the air passing through the evaporator is transmitted to the refrigerant passing through the evaporator in accordance with the size of the air conditioning load, the pressure of the refrigerant gas at the outlet, or downstream end of the evaporator, reflects the size of the air conditioning load.
A swash plate type variable displacement compressor, which has been widely used in vehicles, is provided with a capacity control mechanism, which is operated to hold the pressure of the outlet of the evaporator (hereinafter referred to as the suction pressure (Ps)) to a predetermined target value (hereinafter referred to as the set suction pressure). The capacity control mechanism feedback controls the discharge capacity of the compressor, or the angle of the swash plate, using the suction pressure Ps as a control index such that the flow rate of the refrigerant corresponds to the size of the air conditioning load. A typical example of a capacity control mechanism is an internal control valve. The internal control valve detects the suction pressure Ps with a pressure-sensing member, such as bellows or a diaphragm, and adjusts the pressure (the crank pressure) of a swash plate chamber (or crank chamber) by using displacement of the pressure-sensing member to position a valve body. The position of the valve body determines the angle of the swash plate.
In addition, since a simple internal control valve, which reacts only to the suction pressure, is not able to cope with a demand for minute air conditioning control, a set suction pressure variable type control valve in which the set suction pressure can be changed by external electric control, is needed. For example, a set suction pressure variable type control valve changes the set suction pressure by using an actuator, the force of which is electrically controllable. For example, the actuator may be an electronic solenoid. The actuator increments or decrements the force acting on the pressure-reducing member, which determines the set suction pressure of the internal control valve.
However, in controlling the discharge capacity using an absolute value of the suction pressure as an index, the real suction pressure cannot reach the set suction pressure immediately, even though the set suction pressure is changed electrically. In other words, whether the actual suction pressure follows the change of the set suction pressure responsively depends on the heat load of the evaporator. Therefore, though the set suction pressure is gradually adjusted by the electric control, the change of the discharge capacity of the compressor is delayed or the discharge capacity is not changed continuously and smoothly, and the change of the discharge capacity often becomes rapid.
An objective of the present invention is to provide a control apparatus and a control method of a variable displacement compressor which can improve the control property and responsivity of the discharge capacity.
In accordance with one aspect of the present invention, there is provided a control apparatus for controlling discharge capacity of a variable displacement compressor included in a refrigeration circuit of an air conditioner, said refrigeration circuit including an evaporator, said control apparatus comprising: a differential pressure detector for detecting a differential pressure between two pressure monitoring points set to said refrigeration circuit, on which the discharge capacity of the variable displacement compressor is reflected; a temperature sensor for detecting a cooling state of said evaporator as temperature information; a set differential pressure calculator for calculating a set differential pressure which becomes a control target of a differential pressure between the two pressure monitoring points, based on a temperature detected by the temperature sensor of said evaporator and a target temperature which is a control target of the temperature of said evaporator; a limit value setting device for setting a limit value to the differential pressure between the two pressure monitoring points when the temperature detected by the temperature sensor of said evaporator is lowered from the state higher than a threshold temperature which is set to higher than the target temperature to the state lower than the threshold temperature, and for releasing the setting of the limit value when the temperature detected by the temperature sensor of said evaporator is raised from the state lower than the threshold temperature to the state higher than the threshold temperature; a set differential pressure setting device for comparing the set differential pressure calculated by said set differential pressure calculator with the limit value set by said limit value setting device, for dealing with the set differential pressure in itself if the discharge capacity of the variable displacement compressor which the set differential pressure represents is less than that of the variable displacement compressor which the limit value represents, and for dealing with the limit value as a new set differential pressure if the discharge capacity of the variable displacement compressor which the set differential pressure represents is greater than that of the variable displacement compressor which the limit value represents; and a compressor control mechanism for controlling the discharge capacity of the variable displacement compressor so that the differential pressure detected by the differential pressure detector approaches to the set differential pressure from said set differential pressure setting device.
In accordance with another aspect of the present invention, there is provided a method for controlling discharge capacity of a variable displacement compressor included in a refrigeration circuit of an air conditioner, said refrigeration circuit including an evaporator, said method comprising the steps of: detecting a differential pressure between two pressure monitoring points set to said refrigeration circuit, on which the discharge capacity of the variable displacement compressor is reflected; detecting a cooling state of said evaporator as temperature information; calculating a set differential pressure which becomes a control target of a differential pressure between the two pressure monitoring points based on said temperature information and a target temperature which is a control target of the temperature of said evaporator; setting a limit value to the differential pressure between the two pressure monitoring points when said temperature information is lowered from the state higher than a threshold temperature which is set to higher than the target temperature to the state lower than the threshold temperature, and releasing the setting of the limit value when the detected temperature is raised from the state lower than the threshold temperature to the state higher than the threshold temperature; comparing said set differential pressure with the limit value set, dealing with the set differential pressure in itself if the discharge capacity of the variable displacement compressor which the set differential pressure represents is less than that of the variable displacement compressor which the limit value represents, and dealing with the limit value as a new set differential pressure if the discharge capacity of the variable displacement compressor which the set differential pressure represents is greater than that of variable displacement compressor which the limit value represents; and controlling the discharge capacity of the variable displacement compressor so that the differential pressure approaches to said set differential pressure.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.